The present invention generally relates to the control of the ventilation of laboratory fume hoods and more particularly to an apparatus for controlling the flow of air through a laboratory fume hood to maintain a generally constant face velocity in the uncovered access opening in the front of the fume hood and which utilizes a rotary sensing device for determining the size of the uncovered portion of the access opening.
Fume hoods are used in various kinds of laboratory environments for providing a work place where potentially dangerous chemicals are used. The fume hoods generally comprise an enclosure having at least one movable door that is adapted to cover a front access opening to permit a person to gain access to the interior of the enclosure to conduct experiments and the like. The enclosure is typically connected to an exhaust system for removing any nauseous fumes so that the person will not be exposed to them while performing work in the hood. The sash doors of such fume hoods are designed to be opened either vertically or horizontally and the position of the doors is often referred to as the sash position.
Fume hood controllers that control the flow of air through the fume hood enclosures have become highly sophisticated and are now able to accurately maintain the desired flow characteristics to efficiently exhaust the fumes from the enclosure as a function of the desired average face velocity in the uncovered opening of the fume hood. The average face velocity is generally defined as the flow of air into the fume hood per square foot of open face area of the front access opening of the fume hood, with the size of the open face being dependent upon the position of the sash door or doors. It is highly desirable to minimize the flow of air through the fume hood while providing sufficient flow to ensure a safe environment. It is desirable to minimize the flow for the reason that it is necessary to replenish the air in the room in which the fume hood is located as air is exhausted through the fume hood exhaust duct and the replenishing air must necessarily be conditioned, with such conditioning carrying an attendant cost.
Fume hoods are exhausted by an exhaust system that typically includes a blower that is often capable of being driven at variable speeds to increase or decrease the flow of air from the fume hood to compensate for the varying size of the access opening. Alternatively, there may be a single blower that may or may not be of the type which is driven at variable speeds connected to the exhaust manifold that is in turn connected to individual ducts of multiple fume hoods, and dampers may be provided in the individual ducts to control the flow from the individual fume hoods to the exhaust manifold for the purpose of modulating the flow to maintain the desired average face velocity.
During operation of the fume hood controller, the principal variable that affects the amount of flow through the fume hood is the position of the sash door in the access opening that is typically in the front of the enclosure of the fume hood. Fume hoods may have multiple doors, some of which may be moved horizontally or vertically or both. There have been elaborate electromechanical mechanisms which are installed on the fume hood and sash doors for determining the position of the doors in a reliable manner so that the controller can determine the amount of uncovered area that exists in the access opening at any specific time. When a laboratory worker changes the position of the sash door, there can be a very rapid change in the area of the uncovered access opening which requires the air flow to be dramatically increased to maintain a constant face velocity in the hood. When the sash position is rapidly changed, there is a necessary lag in the system to alter the flow to return the system to its desired average face velocity and the recovery time is a function of the dynamics of the system, including the ability of the sash position sensing portion of the system to provide the correct input to the controller circuitry for the purpose of determining the size of the uncovered opening.
Previously known mechanisms for determining the position of the sash doors have included a relatively elaborate linkage means that was connected to the sash door and rode along a track which varied the resistance value as a function of the position of the sash door. While such an apparatus was reliable, it was located on the front of the cabinet and therefore exposed and vulnerable to being damaged over time. Another prior art mechanism utilized a potentiometer with a string which was connected to the sash and the potentiometer moved through multiple revolutions as the sash door was moved between its fully opened and closed positions. Such a mechanism was often unable to react with sufficient speed and sometimes jammed when a sash door was rapidly moved. This detrimentally affected the response time of the system to regain the desired average face velocity.
Accordingly, it is a primary object of the present invention to provide an improved fume hood controller that can selectively control the flow of air through the fume hood and which utilizes a sash position sensor that is extremely reliable and fast-acting in its operation.
Another object of the present invention is to provide such an improved controller that utilizes a simple acting rotary position sensor that is mounted to the fume hood and which has a simple linkage with the sash door so that an electrical value can be generated that is proportional to the position of the sash door.
Still another object of the present invention is to provide such a controller which is preferably mounted near the top of the fume hood adjacent the door so that electrical signals can be generated that are indicative of the position of the sash door, but which is out of the way from traffic and exposure to physical abuse during normal operation.
Yet another object of the present invention is to provide such an apparatus that is comprised of a relatively few number of parts and which has a simple design which facilitates its installation on laboratory fume hoods of a wide range of designs.
Another object of the present invention lies in the provision for compensating for nonlinearity that results from translating vertical or horizontal movement into rotary movement, with the apparatus of the present invention being capable of compensating for such nonlinear translation to thereby provide signals that are accurately indicative of the size of the uncovered opening.